1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly, to a semiconductor light emitting device having superior current spreading and operation voltage characteristics, and improved reliability.
2. Description of the Related Art
Recently, light emitting devices such as light emitting diodes (LEDs) using group III-V nitride semiconductors (simply, nitride semiconductors) are widely adopted as light sources to obtain blue or green wavelength ranges of light. The nitride semiconductor is a compound semiconductor having a composition expressed by a formula, InxAlyGa(1-x-y)N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. The nitride semiconductor light emitting device typically includes n-type and p-type nitride semiconductor layers grown on a sapphire substrate. Since the sapphire substrate is an insulator, the p- and n-electrodes are disposed on the same side of the substrate (lateral structure). In such a lateral structure of nitride semiconductor light emitting device, there is a problem of current being concentrated at a specific area of the n-type nitride semiconductor layer which is a boundary between a mesa-etched portion and a remaining portion, adjacent to the n-electrode.
FIG. 1 is a sectional view illustrating a conventional nitride semiconductor light emitting device. Referring to FIG. 1, the light emitting device 50 includes an n-type GaN-based semiconductor layer 13, an active layer 15 and a p-type GaN-based semiconductor layer 17 sequentially formed on a sapphire substrate 11. A p-electrode (p-metal) 20 is disposed on the p-type GaN-based semiconductor layer 17, and an n-electrode (n-metal) 30 is disposed on a portion of an upper surface of the n-type GaN-based semiconductor layer 13 exposed by mesa-etching. To form such a mesa structure, mesa etching is conducted in thousands of Å of depth M until the n-type semiconductor layer 13 is exposed.
When operating voltage is applied to the p- and n-electrodes 20 and 30 disposed laterally from each other, current flows through a p-n junction of GaN-based semiconductor and thereby light emission takes place in the active layer. Supposing that the path of the current running on the light emitting device 50 is divided into A, B and C as shown in FIG. 1, relatively greater current flows through and concentrated in path A. The reason for this is that path A has smaller resistance than path B or path C. That is, in path A, the distance of the current passing through the p-electrode of smaller resistance is relatively long whereas the distance of the current passing through the nitride semiconductor layer of greater resistance is relatively short. Resultantly, the current tends to be concentrated in path A of smaller resistance.
As described above, if the current is concentrated in a particular path (path A in which the current passes through the nitride semiconductor layer in the shortest distance), the active layer is not entirely and uniformly utilized. This results in an increase in the operation voltage and a decrease in the luminance. In addition, if the current is concentrated in a particular portion, much heat is generated at that particular portion and the Electrostatic Discharge (ESD) characteristics of the device are degraded. This consequently degrades the reliability of the device. Having the lateral structure in which the two electrodes are disposed on the same side of the substrate, light emitting devices made of not only the nitride semiconductor (GaN-based) but also other types of compound semiconductor (e.g., AlGaAs-based or AlGaInP-based semiconductors) can be adversely affected by such current concentration.